1. Field of the Invention
The present invention relates to a web-fed rotary press having two printing units for printing a printing material web with at least one printing ink at each of the two printing units and at least one drier for drying the printing material web. The present invention also relates to a method for operating such a web-fed rotary press.
2. Description of the Related Art
Various ink systems with different types of drying behavior are used in web-fed rotary presses.
For example, coldset inks are known which are used principally together with absorbent newsprints. These inks dry by the oily constituents of the ink penetrating the papers. This process can be optionally assisted or accelerated by supplying heat.
Heatset inks, which are usually printed on coated papers, are dried under the influence of added heat. The oily constituents of the ink evaporate under the influence of the added heat and are usually suctioned off.
In another category of inks, the ink dries by a chemical reaction which is initiated by irradiation with electromagnetic radiation of a specific wavelength such as, for example, ultraviolet light.
In all cases, suitably designed driers can at least assist the process of ink drying. In the past, these driers have been installed downstream of the printing units.
The aforementioned types of printing inks can experience unwanted smearing or rubbing off. If, for example, in a printing unit that consists of two 9-cylinder satellites, the printing-material web is printed on one side with four inks in the first 9-cylinder satellite, the still wet printing ink on the surface of the impression cylinder of the second 9-cylinder satellite can rub off. For this reason, satellite units have not been used in the past when heatset inks are used. In the case of heatset inks and desired perfecting, blanket-to-blanket cylinder systems are preferably used in the printing units. However, ink smearing problems can arise even in this case with web guide elements, such as turner bars or guide rollers.
Another problem that arises in rotary offset printing is the fan-out effect. In the fan-out effect, the absorption of moisture causes an expansion of the width of the printing-material web. This expansion of the width of the printing-material web can lead to register differences, which can be reduced only by precise register adjustment and by electronic web control. If the distance between two printing units is large, this may even result in a tear of the wet and expanded printing-material web being guided between the two printing units.